1. Field of the Invention
This invention relates to a process for removing calcium from amine-containing solutions. In one aspect, this invention relates to a process for removing calcium from amine-water solutions used in gas purification processing units for removal of H.sub.2 S and CO.sub.2 from gaseous process streams.
2. Description of the Prior Art
It is well known in the gas purification art that sulfur containing compounds can be removed from sulfur-contaminated process gas by contacting such gas with amine absorbent solutions. As used in this specification and the appended claims, the term "amine" means an amine selected from a group consisting of monoethylamine, diethylamine, triethylamine and methyl diethylamine and may include other amines which are used for removal of sulfur-containing compounds from gaseous streams, as are known in the art.
The amine absorbent solutions are effective to absorb H.sub.2 S, as well in some instances other sulfur compounds such as COS and CS.sub.2. The "rich" or sulfur-loaded absorbent is then conventionally stripped by heating to release an acid gas containing H.sub.2 S and other acidic sulfur compounds. The acid gas can be disposed of by incineration; however, incineration alone produces combustion gases having a relatively high SO.sub.2 content, and this is environmentally unacceptable. It is desirable to subject sulfur-containing acid gases to a Claus reaction to recover a substantial portion of the sulfur contained therein. It is also known to be desirable to treat the tail gas stream from the Claus unit in a manner to bring the sulfur emissions from the tail gas unit to an environmentally acceptable range.
Amine-water solutions used for gas treatment operations typically comprise from about fifteen to about fifty percent or more by amine, more typically in the range of about twenty to thirty percent (20-30%) amine, in water. Water used to make an amine-water solution is typically contaminant free demineralized or distilled water. In refinery related operations, water added as make-up water to adjust the solution water concentration for system losses is typically steam condensate, which is substantially demineralized and fairly contaminant-free; however, during H.sub.2 S removal duty and recirculation, such streams will gather hydrocarbon material and contaminants.
Amine-water solutions used as absorbent will entrain or otherwise contain hydrocarbons and heat stable salts which are not removed by stripping during regeneration but generally contain little or no calcium. The presence of even trace amounts of calcium in an amine-water solution causes problems in sulfur removal operations. The calcium readily reacts with the CO.sub.2 present to form calcium carbonate. The calcium carbonate precipitates out at system operating conditions and plugs packing and trays of the contactors, as well as recirculation lines. The plugged vessel components and line requires maintenance downtime and associated cleanout labor to remove the calcium carbonate. Thus, persons operating a gas-scrubbing system employing amine-water solutions will seek to maintain calcium concentration at a very, very low level. In some instances, however, calcium contaminated water enters the system. For instance, if there is a cooler failure and impure cooling water leaks into the amine-water system, the added cooling water can raise calcium to an unacceptable level and precipitated calcium carbonate can become an operational and maintenance problem.